Scanning RSSI receiver system using inverse fast fourier transforms for a cellular communications system with centralized base stations and distributed antenna units

ABSTRACT

A microcellular communications network includes a plurality of base station units and corresponding antenna units. The microcells connected to their respective base station units over a telephone system. The microcells providing received signal strength indication (RSSI) level signals and Supervisory Audio Tone (SAT) frequencies to the base station for hand-in evaluation. The base stations having a scanning receiver for processing the RSSI level signals and SAT frequencies and for performing hand-in for neighboring cells.

FIELD OF THE INVENTION

This invention relates generally to high capacity mobile communicationssystems, and more particularly to digital transport of radio frequencysignals in a microcellular communication system.

BACKGROUND

A conventional cellular phone system 5 is shown in FIG. 1A. Such systemsare currently in widespread use in the United States. As illustrated inFIG. 1A, system 5 has a fixed number of channel sets distributed amongthe base stations 12, 13 serving a plurality of cells 11, 16 arranged ina predetermined reusable pattern. Typical cell areas range from 1 to 300square miles. The larger cells typically cover rural areas and smallercells cover urban areas. Cell antenna sites utilizing the same channelsets are spaced by a sufficient distance to assure that co-channelinterference is held to an acceptably low level.

A mobile unit 10 in a cell 11 has radio telephone transceiver equipmentwhich communicates with similar equipment in base station sites 12, 13as the unit moves from cell to cell. Each base station 12, 13 relaystelephone signals between mobile units 10 and a mobiletelecommunications switching office (MTSO) 17 by way of communicationlines 18. The lines 18 between a cell site and the MTSO 17, typically T1lines, carry separate voice grade circuits for each radio channelequipped at the cell site, and data circuits for switching and othercontrol functions. The MTSO 17 is also connected through paths 19 to aswitched telephone network 15 including fixed subscriber telephonestations as well as various telephone switching offices.

MTSO 17 in FIG. 1A includes a switching network for establishing callconnections between the public switched telephone network 15 and mobileunits 10 located in cell sites 11, 16, and for switching callconnections from one cell site to another. In addition, the MTSO 17includes a dual access feeder for use in switching a call connectionfrom one cell site to another. Various handoff criteria are known in theart and utilize features such as phase ranging to indicate the distanceof a mobile unit from a receiving cell site, triangulation, and receivedsignal strength to indicate the potential desirability of a handoff.Also included in the MTSO 17 is a central processing unit for processingdata received from the cell sites and supervisory signals obtained fromthe network 15 to control the operation of setting up and taking downcall connections.

A conventional base station 12 is illustrated in FIG. 1B. A radiocontroller unit 22 provides the interface between the T1 lines from theMTSO and the base station radio equipment. Transmitters 23, one for eachchannel serviced by the base station, are driven by circuit 22, whichsupplies each transmitter with an analog voice signal. Next, the signalsare passed to a separate nonlinear power amplifier for each channel, orthe signals may be combined and applied to a single linear poweramplifier 24 as shown in FIG. 1B. The output of power amplifier 24 isapplied through duplexer 25 to antenna 26, to be broadcast into thecellular area serviced by the base station.

Signals received in antenna 26 are applied through duplexer 25 tomulti-coupler 27. Multi-coupler 27 applies the wideband signal toreceivers 28 (one for each channel), and scanning receiver 28b. Theanalog voice signal outputs of receivers 28 are applied to circuit 22.Base station 20 may optionally include a diversity antenna 26',corresponding diversity filter 25', multi-coupler 27', and a pluralityof diversity receivers 28', one for each associated main receiver 28.Where implemented, the outputs of diversity receivers 28' are applied tocircuit 22, which would thus include circuitry for selecting thestrongest signal as between corresponding receivers 28 and 28' usingknown techniques. Scanning receiver 28b monitors the strength of signalsin neighboring cells to identify mobiles which are potential candidatesfor being handed into its own cell.

In densely populated urban areas, the capacity of a conventional system5 is severely limited by the relatively small number of channelsavailable in each cell 11, 16. Moreover, the coverage of urban cellularphone systems is limited by blockage, attenuation and shadowing of theRF signals by high rises and other structures. This can also be aproblem with respect to suburban office buildings and complexes.

To increase capacity and coverage, a cell area can be subdivided andassigned frequencies reused in closer proximities at lower power levels.Subdivision can be accomplished by dividing the geographic territory ofa cell, or for example by assigning cells to buildings or floors withina building. While such "microcell" systems are a viable solution tocapacity and coverage problems, it can be difficult to find space at areasonable cost to install conventional base station equipment in eachmicrocell, especially in densely populated urban areas. Furthermore,maintaining a large number of base stations spread throughout a denselypopulated urban area can be time consuming and uneconomical.

AT&T has proposed a system to solve the problem of coverage in urbanareas without having to deploy a large number of conventional basestations. The system is shown and described with respect to FIG. 1 ofAT&T's European Patent Application No. 0 391 597, published on Oct. 10,1990. In that system a grid of antennas sites 40 is placed throughoutthe microcellular system. An optical fiber network 42 interconnects theantennas with the base station 44. Optical wavelength carriers areanalog modulated with RF mobile radio channels for transmission throughthe optical fiber network 26 to the antennas sites 22. A detectorcircuit 27 is provided for each antenna site 22 to receive the modulatedcarrier and reconstruct an RF signal to be applied to the antenna sites22, for transmission into the microcell area 21. RF signals received atantenna sites 22 from mobile units are likewise modulated onto a fiberand transmitted back through optical fiber network 26 to base station25. All of the channels transmitted from base station 25 are distributedto all antenna sites 22. Also, all the channels transmitted from thebase station 25 can be received from the mobile units in any microcell21 and transmitted via optical fiber to base station 25.

The above-described AT&T system has certain limitations. The ability toanalog modulate and demodulate light, the limitations imposed by linereflections, and path loss on the fiber all introduce significantdistortion and errors into an analog modulated signal and thereforelimit the dynamic range of the signals which can be effectively carriedvia an analog system, especially in the uplink direction. These factorslimit the distance from the base station to the antenna sites.

Moreover, in amplitude modulated optical systems an out-of-band signalis required to transmit control and alarm information to and from theantenna sites, again adding to the expense of the modulation anddemodulation equipment.

The problems associated with analog optical modulation are addressed ina system described in pending U.S. patent application Ser. No.08/204,660. In this system, a composite RF signal occupying the entire12.5 MHz cellular band is sampled and digitized by a wideband digitizerat a rate greater than twice the bandwidth of the composite signal (inthis case the sample rate is 30.72 MHz). The digitized signal istransported serially at 552.96 Mbps to a remote site over an opticalfiber using digital optical modulation. The digitized signal is thenconverted back into a replica of the composite RF signal at the remotesite, and amplified for retransmission. The reverse path compositesignal is digitized and transported similarly in the opposite directionover the same fiber using wavelength division multiplexing. Because theoptical modulation is digital, there is no loss of dynamic range overthe fiber.

Both the AT&T analog system and the wideband digital system (describedin the previous paragraph) teach the use of dedicated fiber linesinstalled for each remote antenna site. Alternatives to dedicated fiberlines, including existing telephone circuits, are typically designed tocarry data streams much slower than 552.96 Mbps and the installation ofadditional or higher bandwidth fiber systems is an expensive and timeconsuming undertaking.

The data transferred over the dedicated fiber lines could be compressedby demodulating each channel, transmitting the demodulated signal, andremodulating the signals at the receiving end. Such a system wouldachieve transmission in the limited bandwidth, however, such acompression would not be transparent in the system, since a scanningreceiver would be deprived of the necessary signals to perform hand in.

There is a need in the art for a cellular communication system whichprovides a scanning receiver channel data and hand in data using limitedbandwidth transmission lines. The cellular communication system shouldemploy existing telephone circuits and provide the scanning receiver thenecessary hand in information without an excessive burden on thetransport bit rate and without adding unnecessary complexity to thesystem hardware. The cellular communication system should also providetransparent operation of the existing scanning receiver to preventredundancy in receiver hardware.

SUMMARY OF THE INVENTION

According to one exemplary embodiment of the present invention, there isprovided a microcell system wherein a plurality of commonly locatedmicrocell base station units communicate with a corresponding pluralityof microcell antenna units deployed in respective microcell areas. Eachbase station unit includes conventional RF base station transmitter andreceiver pairs, one for each channel assigned to the microcell.Additional receivers are also provided to receive diversity channels,and to scan channels from neighboring cells. The RF signal outputs fromthe transmitters are combined and applied to a broadbandanalog-to-digital converter. Each microcell unit receives a digitized RFsignal and reconstructs the analog RF signal using a digital-to-analogconverter. The reconstructed RF signal is applied to a power amplifier,the output of which is fed to an antenna for broadcast into themicrocell area.

The antenna units include both a main and a diversity antenna. Theantennas each independently receive RF signals from the mobile units.The RF signal from the main antenna is applied to an analog-to-digitalconverter. A second filter receives the diversity signal from thediversity antenna, and applies that signal to a second analog-to-digitalconverter. The digitized representations of the main and diversitysignals can be transported from the remote location to the base stationvia a high speed digital fiber path.

Thus, the exemplary embodiment outlined above contemplates that themicrocell base station/antenna unit pairs are arranged to provide areusable pattern of channels (as in conventional cellular technology) inthe microcell system. The microcell base station units do not normallyinclude an antenna, and can be located in a convenient and preferablylow cost location, which may be outside of the microcell systemterritory if desired.

In one embodiment, digital signal processing is used to reduce the bitrate, so that the signals can be transported from the remote location tothe base station via the telephone network, rather than a dedicatedfiber. In particular, one embodiment uses a T1 interface between themicrocells and the base station. The base station includes a scanningreceiver for cell operations, such as hand-in. The scanning receiveranalyzes received signal strength indication level (RSSI level)information and generates commands based on the information. Somescanning receivers monitor RSSI levels of adjacent cell clusters and thesupervisory audio tone signalling (SAT signalling) information. Sincethe T1 line lacks the bandwidth needed to transport all baseband voiceand control signals for all neighboring microcells, a system has beendemonstrated which transports only the necessary information for hand-inevaluation, and then reconstructs the wideband signal in order to useexisting scanning receivers at the base station.

In one embodiment, a signal processor at each microcell (or moregenerally, "remote unit") tabularizes and transmits RSSI level and SATfrequency information of neighboring cells to the base station. The basestation decodes this information and performs frequency domain to timedomain conversion and reconstructs a wideband representation of signalsfrom neighboring cells at their corresponding signal level, and withtheir corresponding SAT modulation. The baseband signals of theparticular microcell and the control signals of all cells of interestare thereby added to a composite signal for digital to analogconversion. The analog signal is thereby presented to the scanningreceiver for hand-in processing.

In one embodiment, a complex mixing is performed and a Hilbert TransformFilter is used to generate real outputs. In another embodiment, apositive frequency input to an inverse Fast Fourier Transform moduleprovides real outputs without requiring a Hilbert transform.

In another embodiment, a system featuring frequency modulation of theSAT tones to each of the carriers is described. In one embodiment, fixedfrequency offsets are provided to improve accuracy of center frequencieswithout increasing the size of the inverse fast Fourier transform usedin the system.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and its various features,objects and advantages may be obtained from a consideration of thefollowing detailed description, the appended claims, and the attacheddrawings in which:

FIG. 1A is a functional block diagram of a first prior art mobilecommunications system;

FIG. 1B is a functional block diagram of a prior art base station;

FIG. 1C is a functional block diagram of a prior art microcell mobilecommunications system;

FIG. 2 is a simplified block diagram of an exemplary embodiment of themicrocell communications system of the present invention;

FIG. 3 is a more detailed block diagram of the base station embodimentshown in FIG. 2;

FIG. 4 is a block diagram of one environment in which the present systemmay be implemented, including a base station, host unit, and remote unitof a cellular system;

FIG. 5 is a block diagram of a base station, host unit, and remote unitof a cellular system according to one embodiment of the presentinvention;

FIG. 6 is a spectral diagram showing spectral symmetry about the N/2position;

FIG. 7 is a block diagram showing one embodiment of a FIFO memorycircuit for circular reads of a time domain equivalent of the RSSI levelinformation;

FIG. 8A is a block diagram of a base station, host unit, and remote unitof a cellular system according to one embodiment of the presentinvention;

FIG. 8B is a block diagram of a SAT tone modulation system for RSSIlevel information;

FIG. 9 is a block diagram of one embodiment of a SAT frequencymodulation system for one of the SAT frequencies;

FIG. 10 is a frequency graph of RSSI level information received from theremote units according to one embodiment of the present invention;

FIG. 11 is a frequency graph of a mixing signal at Fs/2 for mixingaccording to one embodiment of the present invention;

FIG. 12 is a resulting frequency graph of the mixing of the RSSI levelinformation in FIG. 10 and the mixing signal information in FIG. 11according to one embodiment of the present invention;

FIG. 13 is a resulting frequency graph of the signal information in FIG.12 after attenuation by a low pass digital filter according to oneembodiment of the present invention;

FIG. 14 is a block diagram of one embodiment of a SAT frequencymodulation system for one of the SAT frequencies, and showing the inputsfor the remaining SAT frequencies and the baseband information;

FIG. 15 is a block diagram of a communication system employing the SATfrequency modulation system of FIG. 14;

FIG. 16 is a block diagram of one embodiment of a SAT frequencymodulation system for one of the SAT frequencies; and

FIG. 17 is a block diagram of a communication system employing the SATfrequency modulation system of FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanying drawings which form apart hereof, in which like numerals refer to like elements throughoutthe several views, and which is shown by way of illustration only,specific embodiments in which the invention may be practiced. It is tobe understood that other embodiments may be utilized and structuralchanges may be made without departing from the scope of the presentinvention.

The general configuration of one exemplary embodiment of the presentinvention is shown in FIG. 2. The microcell system includes a pluralityof microcell areas 100. Deployed within each microcell area 100 is amicrocell remote antenna unit 102. Such units may be deployed on theroof of a building or within a building, or on or in other structures.For example, a microcell antenna unit 102 may be deployed on each floorof a building, on or adjacent an antenna tower, or along a highwaycorridor.

Remote antenna units 102 are connected through fiber 104 or T1 lines (oroptionally another high bandwidth carrier) to respective base stationunits 106. Base station units 106 are interfaced to MTSO 110 over T1lines 112. MTSO 110 is interfaced with a switched telephone network 120,as in a conventional cellular phone system. Microcell base station units106 are preferably located in a single location 114. Such location maybe inside or outside of the area serviced by the microcell system, butin any event is preferably conveniently located for maintenancepurposes.

Referring now to FIG. 3 there is shown a simplified diagram of amicrocell base station 106 according to one exemplary embodiment of thepresent invention. Base station 106 includes conventional transmittersand receivers 23 and 28, respectively, and conventional radio controlleror interface circuitry 22 to the MTSO 110. A digitaltransmitter/receiver unit 130 receives the combined RF signal fromtransmitters 23, digitizes the combined signal and transmits it indigital format over fiber 104A or T1 lines connected to a remote antennaunit 102. Unit 130 also receives a digitized RF signal over fiber 104Bor T1 lines from a remote antenna unit 102, reconstructs thecorresponding analog RF signal, and applies it to receivers 28.Accordingly, conventional equipment may be used on the downstream (MTSO)side of digital transmitting/receiving unit 130.

In one embodiment of the present invention, telephone lines are used tointerconnect microcells to the host equipment and its associated basestation equipment. Telephone lines may offer less bandwidth than otherbroadband communications interfaces, and therefore a system is needed toefficiently exploit the bandwidth of the telephone lines interconnectingthe microcells to their respective base stations.

A fiber microcell is described in pending U.S. patent application Ser.No. 08/204,660, which is hereby incorporated by reference in itsentirety. The 08/204,660 application describes components of a systemwhich uses optical fibers between microcells for transporting cellulartraffic to and from antenna units, and for passively switching. Some ofthe components and operations may be employed in systems usinginterconnections other than optical fibers, and therefore, thedescription of those components and operations are incorporated byreference from the 08/204,660 application.

FIG. 4 is a block diagram showing communications between a remote unit402, a host unit 406, and a cellular base station 408. In oneembodiment, the remote unit is a microcell station, such as a microcell102 in FIG. 2, which has been modified as described herein. In thisexample, the host unit 406 and cellular base station 408 are located atthe base station side of the lines 104 of FIG. 2. The remote unit andhost unit may be constructed so that an existing cellular base stationmay be used, as described in the following examples. Other embodimentsand applications of the present invention exist, and the microcellapplication is intended to be demonstrative and not limiting.

In this implementation a scanning receiver 410 is located at thecellular base station 408. The channel receivers 434 process signalsfrom the remote unit 402 for each channel of the remote unit 402, whilethe scanning receiver 410 processes signals for each channel of eachneighboring microcell. The number of channels to a microcell will bedenoted as "M" throughout this description. In one embodiment, theremote unit 402 includes twenty (20) channels, thus M=20, however, it isto be understood that the number of channels may vary without departingfrom the present invention. In FIG. 4, only one remote unit 402 isshown, however, in a seven cell cluster system, the remote unit 402 willhave six (6) neighbor cells, each having a number of channels (in thisexample, M=20 channels each). This means that the scanning receiver 410may process signals for up to 120 channels of the neighbor cells. Itwill be understood that the present invention will operate on systemswith different numbers of channels and different numbers of neighboringcells per cluster.

In one embodiment, the scanning receiver 410 processes received signalstrength indicator (RSSI) as well as supervisory audio tone (SAT)information from the channels of the neighboring cells of the cluster.Some examples of uses of RSSI level signals and SAT signals is providedin the EIA IS20 AMPS Cellular Standard, which is hereby incorporated byreference in its entirety. However, this is only one example and isprovided to demonstrate some uses and not intended to be limiting orexclusive. Hand-in functions are performed under control of the scanningreceiver 410 based on the RSSI level signals and the SAT signals.

In one embodiment, the SAT signals comprise a set of three continuoustones at 5970 Hz, 6000 Hz, and 6030 Hz. One of these three frequenciesis used by the base station of a given cluster. Neighboring basestation/clusters use one of the remaining two audio tones. The SATsignal of a particular base station is transmitted by the base stationto the mobile. The mobile loops the tone back to the particular basestation. If the received tone is different than the one being sent, theninterference may have occurred. If no tone is returned, then either themobile is fading or its transmitter is off.

By monitoring the RSSI level signals and the SAT signals, the scanningreceiver 410 performs hand-in. Hand-in is the process by which a clusterof cells receives a mobile as it transfers from an adjacent cluster ofcells.

The following description will demonstrate operation of the presentinvention over a T1 telephone line ("T1 link"), however, it is to beunderstood that the present invention may be used on a communicationslink of any bandwidth. For narrowband links, the present system mayexploit the bandwidth of the link to transfer only the necessaryinformation across the link.

One embodiment of the present system is shown in FIG. 4. Cellular basestation 408 transceives signals with remote unit 402 using host unit406. Cellular base station 408 includes a scanning, receiver 410. Someof the signals transferred between the host unit 406 and the remote unit402 are described as follows.

Host unit 406 receives signals from cellular base station 408 andtransfers them to remote unit 402 over T1 link 404. Analog signals fromthe channel exciters 412 are converted to digital signals by thewideband A/D 414 and downconverted by digital downconverter 415 prior toprocessing by digital signal processor demodulator 416. The "xM"designation indicates that the channels (e.g., 20 channels in thisexample) are individually processed by demodulator 416 before beingtransmitted over the T1 line 404 to digital signal processor modulator418. The received digital signals are processed by modulator 418,upconverted by digital upconverter 419, and transformed into an analogwideband signal by wideband D/A 420. In one embodiment, demodulator 416and modulator 418 include a DSP for each channel processed, which isdenoted by the "xM" on FIG. 4. The wideband signals are transmitted bypower amplifier 422 and antenna/duplexer 424.

Host unit 406 receives digital signals from remote unit 402 over T1 link404. Signals received by antenna/duplexer 424 are transformed intodigital representations by wideband A/D 426 and downconverted by digitaldownconverter 427. DSP demodulator 428 processes the digitalrepresentations and transmits the resulting datastream over T1 link 404to DSP modulator 430. In one embodiment, demodulator 428 and modulator430 include one DSP for each channel processed and for each neighboringchannel, as shown by the "x(M+Neighboring)" notation in FIG. 4. Thesignals are then upconverted by digital upconverter 433 and transformedto analog signals by wideband D/A 432 and transmitted to channelreceivers 434 and scanning receiver 410.

In one embodiment, scanning receiver 410 is designed to receive thewideband analog signal and detect the RSSI level and SAT identificationfor each re-modulated signal corresponding to a neighboring cell.

In the embodiment, where remote unit 402 has twenty (20) channels andhas six (6) neighboring cells, each having twenty (20) channels, thescanning receiver 410 processes an analog version of the RSSI levelinformation and SAT frequency information for the 120 channels of theneighboring units to perform hand-in for transceivers moving about thegeographical cellular coverage region. If a T1 link 404 is used totransfer information between the remote unit 402 and host unit 406, thenthe bandwidth of the T1 link 404 and the capacity of the DSP hardware(428 and 430) is insufficient to transfer all of the baseband audioinformation and RSSI level and SAT frequency information for all of thechannels of remote unit 402 and the channels of the neighboring cells.

One approach is to transmit all 120 neighboring channels in the samemanner as the 20 home channels, using a plurality of T1 links andassociated hardware operating in parallel. Such an increase in hardware,however, is impractical in some applications, since it implies a sixfoldincrease in DSP hardware and T1 capacity.

Another approach is a time multiplex system involving the transfer ofone scanning channel at a time, synchronous with the tuning of thescanning receiver 410. However, this approach requires knowledge of theproprietary protocols in the cellular base station 408, or detection ofscanning receiver signalling (e.g., scan receiver LO). This may beimpractical in systems where the scanning receiver 410 is not dedicatedor where the scan function is delegated to multiple receivers.

Another approach is to place the scanning receiver 410 at the remotesite. This way the scanning receiver data is transported to the basestation controller for hand-in functions. This approach also requiresknowledge of base station protocols.

Yet another approach is to transport only the RSSI level informationover the T1 line, and reconstruct unmodulated carriers using an N-pointinverse Fourier transform, as shown in FIG. 5. In this embodiment, DSPdemodulator 528A is programmed to process the channels associated with aparticular remote unit 402, in this example 20 channels. DSP demodulator528B is programmed to detect the RSSI level information for each carrierof the neighboring signals in the digitized wideband signal receivedfrom wideband A/D 426. The channels and the neighboring RSSI informationare multiplexed by multiplexor 529 and transferred as digital data overT1 link 404. The information is demultiplexed by demultiplexer 531 sothe channels are modulated by DSP modulator 530 and the digitized RSSIlevel parameters for the neighboring channels are processed by anN-point inverse fast Fourier transform ("IFFT") 502. The inputs to theIFFT 502 are made symmetric about N/2 in order to ensure a real timedomain output from the N-point IFFT 502. This is shown in FIG. 6. Inthis embodiment, N/2 is greater than the number of channels transformed.The real time domain output is stored in RAM 504. This real time domainoutput may be updated periodically, and need only be updated as fast asRSSI level information is scanned by scanning receiver 410. In oneembodiment, this rate is on the order of a few times a second. If theRAM is read at 30.72 Megasamples/second then the circular read of theRAM constitutes a time domain representation of multiple carriers. Withsymmetric (real) input and inputs on bin centers, the endpoints of thetime domain sample are continuous (i.e., there are no discontinuitieswhen the circular buffer "wraps around").

FIG. 7 shows an alternate memory embodiment, in which an input signal toa switch 702 is used to clock new data into FIFO 704 from FIFO 706. Whenthe input is in a first Boolean state, for example a logical zero (0),then bits from FIFO 706 are clocked into FIFO 704. When the input is asecond Boolean state, for example a logical one (1), then FIFO 704 keepsrecirculating its contents. In one embodiment, the input signal isgenerated from control logic. In another embodiment, a DSP provides theinput signal. Other hardware and/or software may be used to generate theinput signal without departing from the scope and spirit of the presentinvention. Other Boolean state combinations may be used and other memoryembodiments are possible which do not depart from the scope of thepresent invention. For example, one embodiment uses synchronous linebuffers in place of FIFOs. Another example embodiment uses dual-portrandom access memories (RAM) in place of FIFO's. Other memory devicesmay be substituted and combined without departing from the scope andspirit of the present invention.

This solution is acceptable for scanning receivers 410 which onlyrequire RSSI level information, however, some scanning receivers requireboth RSSI level information and SAT frequency information to performhand-in functions.

The next embodiment provides RSSI level information and SAT frequencyinformation for scanning receivers which require both sets ofinformation to process hand-in requests. FIG. 8A shows a generalizedblock diagram of such a system. In the microcell application, signalsfrom antenna/duplexer 424 are received and converted to digital signalsby wideband A/D 426. The downconverted signals are passed to DSPdemodulators 828A and 828B, which process the microcell channels. Inthis embodiment, DSP demodulator 828A is programmed to process Mchannels associated with a particular remote unit 402; in this exampleM=20 channels. DSP demodulator 828B is programmed to process the RSSIlevel and SAT frequency information for each carrier of the neighboringchannels in the digitized wideband signal received from wideband A/D426. The M channels and the neighboring RSSI and SAT frequencyinformation are multiplexed by multiplexer 529 and transferred asdigital data over T1 link 404. The information is demultiplexed bydemultiplexer 531 so the M channels are modulated by DSP modulator 830and the digitized RSSI level parameters and SAT frequency informationfor the neighboring channels arc processed by an N-point inverse fastFourier transform ("IFFT") 802.

In one embodiment, the RSSI level information and SAT frequencyinformation for each neighboring cell site signal are detected andtabularized by DSP demodulator 828B and transported over the T1 link 404with the M channel baseband audio information and control signalinformation processed by DSP demodulator 828A. In one embodiment, foreach active neighboring signal detected by the remote unit, thefollowing information is transmitted to the host site:

    ______________________________________                                        FCC channel number      10 bits;                                              RSSI level (processed by a log function)                                                               8 bits; and                                          SAT state (none, 5970, 6000, 6030)                                                                     2 bits.                                              ______________________________________                                    

Thus a total of 20 bits is communicated to the host site 406. In thisexemplary embodiment, one time slot of a T1 frame is dedicated to thetransport of scanning receiver data. Each T1 time slot is eight bitswide; thus, three consecutive T1 frames are used to transport the datafor each detected neighboring channel. At the host site the RSSI levelinformation is identified by channel number and sorted according totheir respective SAT frequency. These data are received by demultiplexer531 and sent to three N-point IFFTs 802, one for each of the three SATfrequencies.

The time domain equivalent RSSI level information is stored incirculating memory 804. The time domain equivalent RSSI levelinformation is mixed with a carrier of frequency equal to theirrespective SAT signal by mixer 806 and then combined with the M channelbaseband information prior to digital-to-analog conversion by widebandD/A 432. The analog equivalents are received by scanning receiver 410for hand-in evaluation.

FIG. 8B shows a block diagram having three inverse fast Fouriertransforms 812, 822, and 832 for RSSI level data grouped by theirrespective SAT frequencies, such as 5970 Hz, 6000 Hz, and 6030 Hz,respectively. The RSSI level data and their respective SAT frequencydata are detected using DSP demodulators 828A and 828B at the remoteunit 402. The RSSI level data are sorted according to their respectivechannel numbers and corresponding SAT frequencies which were transmittedover T1 link 404 in tabular form. In one embodiment, the RSSI levelinformation is decoded at the host unit 406 by a DSP chip and the RSSIlevel samples arc provided to their respective IFFT algorithms 812, 822,and 832, according to the SAT frequency associated with each channel. Inone embodiment, these algorithms operate in a single DSP. In alternateembodiments dedicated IFFT chips are used. Other hardware and softwarecombinations may be employed without departing from the scope and spiritof the present invention. The number of SAT frequencies may also varywithout departing from the present invention.

The RSSI level information received by the IFFT DSP 802 is processed sothat the N-point IFFTs receive inputs which are symmetric about N/2 toensure the time domain outputs are real. These real time domain outputsare stored in memories 814, 824, and 834, respectively. The time domainoutputs are circularly read and the resulting outputs are blockmodulated with the SAT signal carrier frequency associated with eachinput; 816, 826, and 836, respectively. The final results are summedwith summer 840 and output to the wideband D/A 432 and to scanningreceiver 410 for hand-in processing. In this way, an existing scanningreceiver 410 may be used to decode the wideband signal, using thebandwidth of the T1 link, which is sufficient as only the RSSI levelinformation and SAT frequency information for each neighboring channelare transferred. In this embodiment, the M channel baseband audio andsignalling of the remote unit 402 is summed with the block modulatedsignals to provide the entire wideband signal.

It is to be understood that memories 814, 824, and 834 may be any of theRAM or FIFO embodiments described herein, and may use other memoryelements and combinations without departing from the scope of thepresent invention.

FIG. 9 shows one embodiment of a SAT signal modulation system for one ofthe three SAT frequencies. The remaining SAT frequencies are processedusing the same system. The inputs to the IFFT 910 are the RSSI levelinformation received over the T1 link 404 for a particular SATfrequency. The inputs to the IFFT 910 are symmetrical about the N/2position to provide a real time domain output of the RSSI levelinformation. The output from the IFFT 910 is a real time domain signalin serial digital format which is stored in the memory 912. Memory 912is a RAM in one embodiment. In an alternate embodiment, memory 912 is adual FIFO arrangement. Other memory embodiments are possible withoutdeparting from the scope and spirit of the present invention. The memory912 is circularly read and updated only as often as the scanningreceiver 410 needs an update. A frequency graph of the output of memory912 is shown in FIG. 10.

In the embodiment shown in FIG. 9, the overall sample rate of the systemis twice the sampling rate of the A/D and D/A's of the system, Fs. Theoutput of the memory is mixed with a frequency modulated localoscillator signal at Fs/2 (904) which is mixed with the SAT frequencyfor this signal path, as shown in FIG. 11. The resulting output is thespectrum 906 of FIG. 12. A low pass filter with cutoff at Fs/2 914 isused to attenuate the upper portion of the spectrum 908, as shown inFIG. 13. This output is downsampled by a factor of 2 by downsampler 916.

The sampling rate of the A/D and D/A's, Fs, according to one embodimentof the present invention, is approximately 30.72 Megasample/second,however, the system of FIG. 9 needs to run at 61.44 Megasample/second,which is double Fs. It should be noted that this process inverts thespectrum, requiring the IFFT of the RSSI level information to be loadedin opposite order. Although this embodiment processes real signals, the61.44 Megasample/second rate may exceed the maximum speed of some fieldprogrammable gate arrays (FPGAs).

An embodiment featuring a lower sample rate is shown in FIG. 14. Asingle path for one of the three SAT frequencies is illustrated indetail and its output is labelled RSSI₁. The other paths are modulatedby their respective SAT frequencies and are shown as paths RSSI₂ andRSSI₃, and their processing may be performed using the method describedfor the first path as follows. RSSI level data and their respective SATfrequency data for neighboring channels are detected using a DSPdemodulator at the remote unit 402. The RSSI level data is processed andtransmitted as RSSI level information according to their respectivechannels and their respective SAT frequencies in tabular form over T1link 404. RSSI level information for a particular SAT frequency areinput to the complex IFFT 1410 and a complex output is produced. Thesystem of FIG. 14 operates at Fs=30.72 Megasample/second. This systemuses a complex analytic signal, e^(jx), where x is a signal from source1420 (having the same frequency as the particular SAT frequency ofinterest), and mixed with the complex time domain RSSI weighted carriersoutput from complex IFFT 1410. The output from the complex mix isseparated into its real and imaginary components by convertor 1414. Theimaginary component is delayed using delay 1416 and the real componentpasses through a Hilbert Transform 1418. The resulting outputs aresubtracted by subtractor 1424 and the final output RSSI₁ is then summedwith the other SAT frequency paths RSSI₂, and RSSI₃ and the M channelbaseband audio information and signalling information for the remoteunit 402. The information is then sent to the wideband D/A 432 andultimately to the scanning receiver 410 for hand-in evaluation andprocessing.

The use of complex math allows the mixing with a baseband FM analyticcarrier while maintaining the ability to separate positive and negativefrequencies. After the mix, the positive and negative frequencies arenot symmetric and the modulation is 180 degrees out of phase. TheHilbert Transform Filter 1418 is used to isolate the positivefrequencies. The inserted delay 1416 is used to match the delay of theHilbert Transform Filter 1418 which is, in one embodiment, a finiteimpulse response (FIR) implementation.

A larger scale drawing of the implementation of FIG. 14 is shown in FIG.15. The components in FIG. 14 were shown in FIG. 15 for each SAT tonewith the components used with the first SAT tone having an "A" suffix,the components used with the second SAT tone having a "B" suffix, andthe components used with the third SAT tone having a "C" suffix. Asdescribed earlier in one embodiment, the RSSI level data for theneighboring channels are detected at the DSP demodulators at remote unit402 and transferred via a tabular format to the host unit 406. The RSSIlevel information for each particular SAT frequency are separated andsent to their respective complex IFFT, 1410A, 1410B, and 1410C. Thebaseband information and time domain RSSI weighted carriers modulatedwith their particular SAT information are combined and transferred tothe scanning receiver 410 for hand-in evaluation.

Another embodiment replaces the frequency modulated complex analyticsignal with a circularly read memory. In one embodiment, the samples ofthe modulated e^(jx) function are in a form that repeats in an integralnumber of 30.72 MHz samples (or whatever sampling frequency, Fs, thesystem uses). The output is e^(jx) =Cos(ω_(m) t)+j Sin(ω_(m) t), whereω_(m) =2×π×5970, 6000, or 6030 Hz. These frequencies can be representedto within 1 Hz by a waveform which repeats in the following number ofintegral samples:

    ______________________________________                                        No. of Samples @ 30.72 Mhz                                                                      Actual Frequency                                            ______________________________________                                        746               5969.69                                                     720               6000.00                                                     695               6029.44                                                     ______________________________________                                    

Note that when calculating the e^(jx) term the Cos(ω_(m) t)and Sin(ω_(m)t) terms are identical except for a 90 degree phase difference. Thismeans that for each SAT frequency, an identical table of values is usedto generate the e^(jx) output. In one embodiment a single table isgenerated and stored in memory for each SAT signal. The values are thencircularly read out of the memory with a 90 degree phase difference toaccount for the Sin and Cos phase difference.

Yet another embodiment provides the SAT frequency modulation of the RSSIlevel information without having to use a Hilbert Transform Filter. TheHilbert Transform Filter was needed because of the presence of positiveand negative frequencies at the output of the IFFT. The HilbertTransform Filter becomes unnecessary if negative frequencies areeliminated at the IFFT. In a complex IFFT, the negative frequencies areeliminated by zero padding the inputs above N/2.

For a single carrier of frequency ω_(c), the complex IFFT output becomesCos(ω_(c) t)+j Sin(ω_(c) t), wherein ω_(c) represents the positivefrequency component only. This output is then complex multiplied withthe SAT modulated 0 Hz carrier, which is Cos(β Sin(ω_(m) t))+j Sin(βSin(ω_(m) t)). However, since only the real output from the complexmultiply is needed, the resulting equations require only 2 realmultiplies: ##EQU1## which is a real output.

An embodiment combining the zero padding/positive frequency concept withthe two (2) multiply concept is shown in FIG. 16. In one embodiment, theIFFT 1602 output is calculated at various times and the outputs arestored in memory. The SAT modulated sources 1604 and 1606 are alsostored in memory. Both memories are read circularly at a 30.72 MHz rate(or whatever sampling frequency is used). The multiplies and adds alsotake place at that rate.

Therefore, the implementation of one embodiment of the present system isas follows:

1. At the remote unit 402, detect the SAT frequency data and RSSI leveldata of each scanned channel.

2. Transmit the SAT frequency information and RSSI level information forall scanned channels to the host unit 406.

3. Sort the RSSI level information and channel numbers corresponding toeach of the three SAT frequencies (e.g., a 3 SAT tone embodiment).

4. For each SAT frequency:

A. construct a one sided (positive frequency only) representation of thefrequency domain spectrum of the carriers corresponding to that SATfrequency.

B. calculate the complex IFFT and store the real and imaginary timedomain result in RAM (to be updated periodically, or at a ratecommensurate with the scanning receiver 410).

C. calculate the modulated zero Hz complex carrier component waveforms,Sin(β Sin(ω_(m) t)) and Cos(β Sin(ω_(m) t)) and store in memory (may becalculated once and need not be updated).

D. Calculate the RE{IFFT}×Cos(β Sin(ω_(m) t))-IM{IFFT}×Sin(β Sin(ω_(m)t)) at a 30.72 MHz rate.

5. Sum outputs corresponding to all 3 SAT frequencies with the outputsfrom the traffic channel synthesizers. In this example, the trafficchannel synthesizers produce synthesized replicas of the 20 channels (Mchannels) identified with a given cell.

FIG. 17 shows a block diagram of a system having circulating memoriesfor providing the time domain RSSI information and the necessarycomponents of the analytic zero Hertz carrier according to oneembodiment of the present invention. The Hilbert Transform Filter is notnecessary, since the positive frequency only transform is implemented.Further, this system operates at Fs, since a complex approach is takenas described above. In one embodiment, tabularized RSSI levelinformation and SAT frequency information are sent over T1 line 404 andreceived at host unit 406 to be demodulated by demodulator 531. Thebaseband information on M channels is sent to DSP modulator 830 andupconverted for summation to create a version of the original broadbandsignal for scanning receiver 410. The RSSI level information is sent bydemodulator 531 to IFFTs 1602A-C, based on the SAT frequency associatedwith the RSSI level information. In this embodiment, memories 1730A-Ccircularly provide the time domain RSSI information for mixing with theSin(β Sin(ω_(m) t)) and Cos(β Sin(ω_(m) t)) components for each SATfrequency. The Sin(β Sin(ω_(m) t)) and Cos(β Sin(ω_(m) t)) componentsfor each SAT frequency are circularly provided by memories 1740A-C,respectively. As described above, in an alternate embodiment, a singletable of the needed Sin and Cos samples may be stored and accessed asneeded, since the Sin and Cos components are only ninety degrees out ofphase. Furthermore, in one embodiment the Sin(β Sin(ω_(m) t)) and Cos(βSin(ω_(m) t)) components need only be calculated once, and the IFFToutputs are calculated about as often as the scanning receiver 410processes RSSI level information. In other embodiments, the IFFT outputsare calculated at different rates without departing from the scope andspirit of the present invention.

Systems have been described which reproduce carriers that are centeredexactly on the frequency bins of the IFFT input. This is useful topreserve the simplicity of the IFFT input, and to maintain continuity atthe endpoints of the time domain waveform. The ability to reproducecarriers at the correct frequency is thus limited by the resolution ofthe N-point IFFT. For example, at a 30.72 MHz sample rate, theresolution is 30 kHz for N=1024, 15 kHz for N=2048, etc. (The FFTalgorithm requires N to be a power of 2).

While any resolution better than 30 kHz is sufficient to match the 30kHz channel spacing in the AMPS cellular band, there is the possibilitythat the frequency of the local oscillator used to upconvert thecarriers from baseband to RF will result in all the carriers beingoffset from the desired locations by a fixed amount. While theresolution can always be doubled by doubling N, this presents a burdenon the computational and storage requirements of the DSP. A system forcorrecting frequency offset without arbitrarily increasing N isdescribed herein.

With reference to FIG. 16, a frequency shift is introduced into the SATmodulation waveforms (1604 and 1606) by replacing Sin(β Sin(ω_(m) t))and Cos(β Sin(ω_(m) t)) with Sin(ω_(s) t+ωβ Sin(ω_(m) t)) and Cos(ω_(s)t+ωβ Sin(ω_(m) t)), respectively, where ω_(s) is the desired frequencyshift. The output of the algorithm in FIG. 16 then becomes Cos[(ω_(s)+ω_(c))t+β Sin(ω_(m) t)], where ω_(c) is the closest estimate of thedesired carrier frequency allowed by the resolution of the IFFT, andω_(s) is the amount of frequency shift needed to move the carrier closerto the actual desired frequency.

The resulting modulating waveform (1604' and 1606') now has twofrequency components (ω_(m) and ω_(s)), which may substantially increasethe amount of memory needed to store a repeating portion of thatwaveform. However, if ω_(s) =n×ω_(m), where n is an integer, then thewaveform will repeat in one ω_(m) period, and the memory requirementsare not affected. On the other hand, for example, frequency shifts ofn×1/2ω_(m), where n is an odd integer, would double memory requirements.

As an example, an N=1024 IFFT may result in carriers which fall 10 kHzbelow the desired frequency locations. Incorporating a frequency shiftof ω_(s) =2×ω_(m) (a shift of 12 kHz, for a SAT of 6000 Hz) reduces thenet error to approximately 2 kHz. As another example, an N=2048 IFFT mayresult in carriers which fall 5 kHz above the desired locations.Incorporating a frequency shift of ω_(s) =-1×ω_(m) (a shift of -6 kHzfor a SAT of 6000 Hz) reduces the net error to approximately -1 kHz.

The description of embodiments herein is not limited to a three SATfrequency system. Other embodiments incorporating different numbers ofSAT frequencies may be constructed using the teachings presented herein,and these embodiments do not depart from the scope and spirit of thepresent invention.

The invention(s) has been described in detail, and those of skill in theart will recognize that many modifications and changes may be madethereto without departing from the spirit and the scope of the presentinvention. For example, the sampling rates and structures may differwithout departing from the present invention. Furthermore, theembodiments described herein are not intended in an exclusive orlimiting sense, and that the invention is as claimed in the followingclaims and their equivalents.

What is claimed is:
 1. A method for communicating in a cellular network,the cellular network including one or more remote cells and a host unit,the method comprising the steps of:transmitting messages over atelephone line, the messages comprising baseband signals and receivedsignal strength indication levels for each channel of a particularremote unit, the messages further comprising received signal strengthindication levels for one or more channels of one or more neighboringremote units, the messages identifying an associated supervisory audiotone frequency for each channel; and at the host unit, constructing abroadband signal recognizable by a scanning receiver, the broadbandsignal including:for each channel of the particular remote cell, acarrier modulated by the baseband signals and the supervisory audio tonefrequency, and scaled by its associated received signal strengthindication level; and for each channel of neighboring cells, a carriermodulated by the supervisory audio tone frequency, scaled by itsassociated received signal strength indication level; where the step ofconstructing includes the steps of:performing a separate transform onthe associated received signal strength indication levels for eachsupervisory audio tone frequency to generate associated time domainreceived signal strength indication weighted carriers; for eachsupervisory audio tone frequency, modulating a carrier having afrequency related to the supervisory audio tone frequency with theassociated time domain received signal strength indication weightedcarriers to generate a set of supervisory audio tone modulated signals;and combining the modulated signals corresponding to each supervisoryaudio tone frequency with the baseband signals to construct thebroadband signal.
 2. The method of claim 1, where the step of performinga separate transform further comprises the step of:generating a separateinverse Fourier transform on a positive frequency representation of theassociated received signal strength indication levels for eachsupervisory audio tone frequency to generate the associated time domainreceived signal strength indication weighted carriers.
 3. The method ofclaim 2, where the positive frequency representation is generated byzero padding the input to the separate inverse Fourier transforms abovethe N/2 position.
 4. The method of claim 2 further comprising the stepof storing the associated time domain received signal strengthindication weighted carriers in a memory.
 5. The method of claim 1,where the step of modulating further comprises the steps of, for eachsupervisory audio tone frequency:synthesizing a complex signal, e^(jx),where x is a tone with frequency approximately equal to a frequency ofthe supervisory audio tone frequency, to generate a complexrepresentation of a frequency modulated zero hertz signal; and mixingthe complex representation with the associated time domain receivedsignal strength indication weighted carriers to generate a modulatedsignal.
 6. The method of claim 5, where the step of synthesizingcomprises the steps of:generating a real component and an imaginarycomponent of the complex representation; and storing the real componentand the imaginary component in a memory.
 7. The method of claim 1, wherethe step of performing a separate transform further comprises the stepsof:generating a separate inverse Fourier transform on a positivefrequency representation of the associated received signal strengthindication levels for each supervisory audio tone frequency to generatethe associated time domain received signal strength indication weightedcarriers; and storing the associated time domain received signalstrength indication weighted carriers in a memory; where the step ofmodulating further comprises the steps of, for each supervisory audiotone frequency:synthesizing a complex signal, e^(jx), where x is a tonewith frequency approximately equal to a frequency of the supervisoryaudio tone frequency, to generate a complex representation of afrequency modulated zero hertz signal; and mixing the complexrepresentation with the associated time domain received signal strengthindication weighted carriers to generate a modulated signal.
 8. Themethod of claim 7, where the time domain received signal strengthindication weighted carriers are stored in memory in separate imaginaryand real components, and the complex representation is stored in memoryin separate imaginary and real components, and where the step of mixingcomprises the steps of:multiplying the real components of the receivedsignal strength indication levels and of the complex representation tocreate a first result; multiplying the imaginary components of thereceived signal strength indication levels and of the complexrepresentation to create a second result; subtracting the second resultfrom the first result to yield the modulated signal.
 9. The method ofclaim 8, where the received signal strength indication levels areprocessed at a rate approximately equal to a rate at which the scanningreceiver reads received signal strength indication information.
 10. Themethod of claim 8, where the complex representation is calculated once.11. The method of claim 9, where the complex representation real andimaginary components are generated from one coefficient table.
 12. Themethod of claim 7, further comprising the steps of:generating amodulated signal for each supervisory audio tone frequency; adding eachmodulated signal to a baseband signal to construct the broadband signal.13. The method of claim 8, where the memory is a RAM.
 14. The method ofclaim 8, where the memory is a FIFO buffer.
 15. The method of claim 7,where the time domain received signal strength indication weightedcarriers are stored in memory in separate imaginary and real components,and the complex representation is stored in memory in Cos(β Sin(ω_(m)t)) and Sin(β Sin(ω_(m) t)) components, and where the step of mixingcomprises the steps of:multiplying the real component of the receivedsignal strength indication levels and the Cos(β Sin(ω_(m) t)) componentto create a first result; multiplying the imaginary component of thereceived signal strength indication levels and the Sin(β Sin(ω_(m) t))component to create a second result; subtracting the second result fromthe first result to yield the modulated signal.
 16. The method of claim7, where the time domain received signal strength indication weightedcarriers are stored in memory in separate imaginary and real components,and the complex representation is stored in memory in Cos(ω_(s) t+ωβSin(ω_(m) t)) and Sin(ω_(s) t+ωβ Sin(ω_(m) t)) components, and where thestep of mixing comprises the steps of:multiplying the real component ofthe received signal strength indication levels and the Cos(ω_(s) t+ωβSin(ω_(m) t)) component to create a first result; multiplying theimaginary component of the received signal strength indication levelsand the Sin(ω_(s) t+ωβ Sin(ω_(m) t)) component to create a secondresult; subtracting the second result from the first result to yield themodulated signal,where ω_(s) is a frequency shift to offset to a desiredfrequency.
 17. A communications system, comprising:a base stationconnected to a host unit, the host unit connected to a scanningreceiver; and one or more remote units, connected to the host unit overa telephone line, the remote units transmitting tabularized receivedsignal strength indication levels and supervisory audio tone frequencyinformation; where the host unit processes the tabularized receivedsignal strength indication levels and supervisory audio tone signallinginformation to generate a broadband signal for the scanning receiver;where the host unit comprises frequency domain to time domain convertorsand modulators for generating the broadband signal from the tabularizedreceived signal strength indication levels and supervisory audio tonesignalling information.
 18. A communications system, comprising:a basestation connected to a host unit, the host unit connected to a scanningreceiver; and one or more remote units, connected to the host unit overa telephone line, the remote units transmitting tabularized receivedsignal strength indication levels and supervisory audio tone frequencyinformation; where the host unit processes the tabularized receivedsignal strength indication levels and supervisory audio tone signallinginformation to generate a broadband signal for the scanning receiver;and where the host unit comprises a digital signal processor forgenerating a complex signal, modulating the complex analog signal with aparticular supervisory audio tone frequency, and signal addition meansfor adding signals to construct the broadband signal.
 19. Thecommunications system of claim 17, where the host unit comprises one ormore memories for storing components used in generating the broadbandsignal.
 20. A communications system, comprising:a Hilbert transformfilter; and a base station connected to a host unit, the host unitconnected to a scanning receiver; and one or more remote units,connected to the host unit over a telephone line, the remote unitstransmitting tabularized received signal strength indication levels andsupervisory audio tone frequency information, where the host unitprocesses the tabularized received signal strength indication levels andsupervisory audio tone signalling information to generate a broadbandsignal for the scanning receiver.
 21. A communications system,comprising:a host unit, connected to a scanning receiver, the host unitprocessing received signal strength indication levels and supervisoryaudio tone signalling information to generate a broadband signal for thescanning receiver, and where the host unit comprises frequency domain totime domain convertors and modulators for generating the broadbandsignal from the received signal strength indication levels andsupervisory audio tone signalling information.
 22. A communicationssystem, comprising:a host unit, connected to a scanning receiver, thehost unit processing received signal strength indication levels andsupervisory audio tone signalling information to generate a broadbandsignal for the scanning receiver, and where the host unit comprises adigital signal processor for generating a complex signal, modulating thecomplex analog signal with a particular supervisory audio tonefrequency, and signal addition means for adding signals to construct thebroadband signal.
 23. The communications system of claim 21, where thehost unit comprises one or more memories for storing components used ingenerating the broadband signal.
 24. A communications system,comprising:a host unit, connected to a scanning receiver, the host unitprocessing received signal strength indication levels and supervisoryaudio tone signalling information to generate a broadband signal for thescanning receiver; and a Hilbert transform filter.